recursive to iterative (java)

as a beginner in programming I am trying to convert the following recursive method to an iterative one but I just don't get the hang of it. The method or function has a binary tree like recursion and I would like to use an array for the iterative solution.. unfortunately I am very confused how to do it.

I have already checked the way of converting the fibonnaci recursive method to an iterative one. But I think this is not the same here. Also I am not sure if a tree search method is useful?! Any help, hint, idea would be appreciated. Thanks.